Step 1

In microwavable measuring cup or bowl, stir 1⁄3 cup water and sugar and microwave on High until warm, 30 to 45 seconds. Stir until sugar dissolves. (Alternatively, in small saucepan, stir 1⁄3 cup water and sugar over medium heat just until sugar dissolves.) In blender, add sugar syrup, grapes, grappa, and lemon juice and blend on high speed until smooth, about 1 minute. Pour into 13-by-9-inch baking pan.

Step 2

Freeze granita until frozen along edges, about 1 hour. With fork, scrape ice along edges into center of pan. Return to freezer. Repeat stirring or scraping with fork every hour until frozen, 4 to 5 hours. Using fork, scrape granita into bowls.

Step 3

Serving size: about 3⁄4 cup